NASA / TM--2002-211482

U.S. ARMY

ARL-TR-2867

RESEARCH LABORATORY

A Systems Approach to the Solid Lubrication of

Foil Air Bearings for Oil-Free Turbomachinery

Christopher DellaCorte and Antonio R. Zaldana

Glenn Research Center, Cleveland, Ohio

Kevin C. Radil

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

October 2002
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NASA/TM--2002-211482

U.S. ARMY

ARL-TR-2867

RESEARCH LABORATORY

A Systems Approach to the Solid Lubrication of

Foil Air Bearings for Oil-Free Turbomachinery

Christopher DellaCorte and Antonio R. Zaldana

Glenn Research Center, Cleveland, Ohio

Kevin C. Radil

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

National Aeronautics and

Space Administration

Glenn Research Center

October 2002
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Available electronically at http://gltrs.grc.nasa.gov
A Systems Approach to the Solid Lubrication of Foil Air Bearings

for Oil-Free Turbomachinery

Christopher DellaCorte and Antonio R. Zaldana

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Kevin C. Radii

U.S. Army Research Laboratory

Glenn Research Center

Cleveland, Ohio 44135

Abstract

Foil air bearings are self-acting hydrodynamic bearings which rely upon solid lubricants to reduce

friction and minimize wear during sliding which occurs at start-up and shut-down when surface speeds

are too low to allow the formation of a hydrodynamic air film. This solid lubrication is typically

accomplished by coating the non-moving foil surface with a thin, soft polymeric film. The following

paper introduces a systems approach in which the solid lubrication is provided by a combination of self

lubricating shaft coatings coupled with various wear resistant and lubricating foil coatings. The use of

multiple materials, each providing different functions is modeled after oil-lubricated hydrodynamic sleeve

bearing technology which utilizes various coatings and surface treatments in conjunction with oil

lubricants to achieve optimum performance.

In this study, room temperature load capacity tests are performed on journal foil air bearings

operating at 14,000 rpm. Different shaft and foil coating technologies such as plasma sprayed composites,

ceramic, polymer and inorganic lubricant coatings are evaluated as foil bearing lubricants. The results

indicate that bearing performance is improved through the individual use of the lubricants and treatments

tested. Further, combining several solid lubricants together yielded synergistically better results than any

material alone.

Introduction

Blok and Van Possum published the first paper on foil bearings in 1953 (ref. 1). Although they

coined the term "foil bearing," their work actually concerned an oil lubricated shaft running against an

acetate film or "foil." The concept of a flexible bearing surface and its implications to and potential for

improved capabilities was quickly adapted by other technologists and papers on air lubricated foil

bearings began to appear in the open literature in the following decade (refs. 2 and 3).

Foil air bearings are similar to conventional oil-lubricated bearings in that the fluid film pressure is

developed via the hydrodynamic effect. Unlike conventional bearings, foil air bearings use air as their

working fluid and the bearing surface is compliant rather than rigid. This compliant inner or top foil

surface is supported by a spring pack or bump foil layer which allows the bearing to accommodate shaft

misalignment, thermal and centrifugal distortion, the presence of wear debris and also allows the designer

to tailor the operational foil shape to enhance film pressure and hence bearing load capacity (ref. 4).

Figure 1 schematically shows typical foil bearing designs. Micro-sliding, which occurs between the top

foil and its spring support and within the spring foils, contributes significant coulomb damping properties

NASA/TM 2002-211482 1
tothebearing(ref.5).Sincefoilbearingsdonotuseoilastheirworkingfluidtheycanandareroutinely

usedoveranextremelywidetemperaturerange,fromcryogenictoover650°C,notpossiblewithoil

lubrication.

Foilairbearings, however,dorequiresolidlubricationtopreventwearandreducefrictionatverylow

speedsencounteredduringstart-upandshut-downpriortothedevelopment ofhehydrodynamicgasfilm

andalsoduringmomentarybearingoverloadssuchashighspeedrubs(ref.6).Traditionally, thissolid

lubricationisprovidedbyapplyingathinpolymerfilmorcoatingtothefoil surface.Thisapproachworks

wellunlessthecoatingwearsthroughresultinginhighfrictionandwear.Also,traditionalsolidlubricants

(graphite,PTFE,MoS2),aretemperaturelimitedtoonlyabout300°C(ref.7).Forhighertemperatures

andlongerlifealternateapproaches mustbeconsidered.

Oneapproachundertakenattheauthors' laboratoryistocoatherotatingshaftsurfacewithahigh

temperaturesolidlubricantcompositecoating(ref.8).Thistechniqueovercomestheproblemoflocalized

lubricantcoatingfailureonafoil causingexcessivefrictionandwear.Whentheshaftiscoated,the

rubbingsurfaceofthefoil iscontinuallyexposedtotheentireshaftsurfacetherebyreducinglocalwear

spotsthatwouldformathighlyloadedregionsonthefoil.Byusingtheshaftcoatingasareservoirfor

lubricants, alubricioustransferfilmcanformonthefoil surface,in-situ,fuxtherimprovingtribological

properties. OneexampleofthisapproachisthePS304coating.Reportedonextensivelyintheliterature,

PS304isaplasmasprayedcompositecoatingmadefromapowderblendof 60wt%NiCr,20wt%Cr203,

10wt%Ag,and10wt%BaF2/CaF2eutectic(refs.9to 11).Eachconstituent inPS034performsaunique

function.TheNiCractsasaductilematrixorbinder,thechromeoxideasawearresistant hardenerand

thesilverandeutecticserveaslowandhightemperaturesolidlubricantsrespectively.

Whenappliedtotheshaftsurfaceof afoilbearingsystemandsubjectedtorepeatedstart/stopcycles,

thesolidlubricantsformasoftsurfacelayer,whichalsotransferstothefoil surface.Duringhigh

temperatureoperation, metaloxidesfromthesuperalloyfoilandtheNiCrbinderphaseinthePS304

coatingalsocontributetotheformationofcomplex,glassy,lubriciousurfacefilms,whichprovidesolid

lubrication.Reference11reportsonpostestsurfaceanalysesofPS304lubricatedfoilbearingsrunat

discretetemperatures from25to650°C.Atdifferentoperatingtemperatures surfacefilmswithdifferent

chemical compositionsdevelopontheshaftandfoil surfaces. Althoughtheirchemistrydiffers,these

filmsallprovidethesametypeof solidlubricationeffect.Infact,PS304lubricatedbearingshave

demonstrated liveswellinexcessof 100000start/stopcycles;greatlyexceedingtherequirementsof

allforeseenapplications(ref.12).

Thesuccessful demonstrationflonglived,hightemperaturefoilairbearingshaspromptedthe

initiationofresearcheffortsto extendtheapplicationof foilairbearingstolargeroil-freeturbomachinery

systems. Foilairbearingsoperatingatlowtemperatures utilizingpolymerfoilcoatingshavebeenin

commercial serviceinaircyclemachinesforaircraftcabinpressurizationadcoolingforoverthree

decades(refs.3and13).Intheseapplicationstheoil-free,maintenancefrecharacteristics offeredbyfoil

bearingsiresponsibleforobservedmeantimebetweenfailures(MTBF's)of over100000hours.Low

costandquietoperationwithouthepossibilityofoil contaminationf cabinairhavemadefoilbearings

thepreferredshaftsupporttechnologyinthesemachines. Morerecently,smalloil-freemicroturbine

electrical generatorshavebeencommercializedusingfoilbearings. Again,polymerorotherthinfilm

coatingsformthebasisforthesolidlubricationofthebearings.Asintheaircyclemachineapplication,

theprimarybenefitofoil-freetechnologyisthelonglifeandmaintenancefrecharacteristics ofthe

bearings.

Thefirstcommerciallyavailableoil-freemicroturbinetobemarketedproduces30kWof electrical

poweratanenginespeedof 96000rpm(ref.14).Thiswasfollowedshortlythereafterbyacompetitor

witha75kWsystemusingalargerenginerotatingatsomewhat lowerspeeds. Morerecently,several

othercompaniesaretestingevenlargerprototypeunitswithpoweroutputlevelsexceeding200kW.

Basedupontheseactivitiesandadditional technological developments thereisarenewedinterestin

developingoil-freegasturbineaircraftenginesutilizingadvancedhightemperaturefoilbearingsand

solidlubricantslikePS304.Tosupportthecontinuingdevelopment ofoil-freeaircraftengines,research

NASA/TM2002-211482 2
isbeingconductedonbearingdesignanddevelopment, rotordynamicsystemmodelingandsolid

lubrication.Thispaperdescribesanefforttofuxtherimprovethesolidlubricationperformanceof foil

bearingsoperatingagainstPS304coatedshafts.

RadilrecentlypublishedapaperthatshowsthatPS304lubricatedfoilairbearingsexhibit

significantlyreducedloadcapacitiesintheir"as-installed" conditionpriortobeingsubjectedto abreak-in

procedureofrepeatedhightemperaturestart-stopcycles(ref.16).Hefoundthatunworn,as-installed

bearingshadrelativelyroughshaftandfoil surfacesthathadnotyetdevelopedbeneficial,lubricantrich

surfacetransferlayersthatformastheresultofthelowspeedslidingwhichoccursduringrepeated

start/stopcycling.Withoutthepresenceofpolishedsurfacesandlowfrictionsurfacefilms,highstarting

torqueandreducedloadcapacityresulted.Hisworkshowedthatimprovingthetribological properties,

namelythesmoothness and/orsolidlubricatingqualities,dramaticallyenhancedbearingloadcapacity.

Foilbearingloadcapacityisexpressedinrelationtoabeating'sloadcapacitycoefficient, D. This

coefficient, defined fully in reference 17, is an empirically established performance parameter which

relates bearing size and speed to the load that a bearing can support. Mathematically, it is defined as

follows:

W = D (Lxd) (dxKrpm)

where:

Wis the maximum steady load that can be supported, N (lbs)

D is the bearing load capacity coefficient, N/mm 3xKrpm (lbs/in. 3xKrpm)

L is the bearing axial length, mm (in.)

d is the shaft diameter, mm (in.)

Krpm is the shaft rotational speed in thousands of revolutions per minute (Krpm)

This relationship can be remembered easily for advanced technology bearings, which have

performance coefficients of around 1.0 when non-SI or English units are used. An advanced design

(designated Generation III in ref. 17) foil bearing, like the type depicted in figure 2, will support about

"one pound of load per in. 2 of projected bearing area per inch of bearing diameter per thousand rpm." The

earliest foil bearing designs, designated Generation I, had very simple elastic support structures (spring

systems) and exhibited load capacity coefficients of around 0.3. The development of more complex

bearing designs in which the elastic foundation varied circumferentially or axially are defined as

Generation II and exhibit load capacity coefficients around 0.5. The most recent bearing designs have

elastic structures which tailor the spring foundation both circumferentially and axially, are designated

Generation III bearings, and have load capacity coefficients of about 1.0. Foil bearings used in air

cycle machines are Generation I bearings. Generation II bearings have been used successfully in

turbocompressors and small microtuxbines. It is expected that Generation III bearings with load

capacity coefficients near 1.0 will be required for aircraft engine applications.

The Generation III bearings lubricated with PS304 shaft coatings produce load capacity coefficients

of 0.9 to 1.0 from 25 to 650 °C after they have been broken-in over repeated start/stop cycles at elevated

temperatures (ref. 12). When the load capacity coefficient was measured for these same bearings in the

as-installed condition the load capacity coefficient, D, at 25 °C was only 0.3, a three fold reduction

(ref. 16). When a new foil bearing was fitted to a broken-in, PS304 coated shaft, the load capacity

coefficient, D, rose to about 0.5. Similarly, if a previously broken-in foil bearing was fitted to a newly

ground PS304 coated shaft the coefficient was 0.6. Clearly, improving the tribological properties of each

surface improves bearing performance, and improving both mating surfaces offers fuxther improvements.

Theoretically, the load capacity of a foil air bearing is governed by its design, operating condition

(speed and temperature) and the foil and shaft surface roughness. Load capacity is reached when the

hydrodynamic gas film pressure forces no longer creates a gas film thickness greater than the sum of the

average surface roughness of the foil and shaft. Higher loads, or lower speeds, reduces the air film

thickness allowing rubbing contact between asperities (high points) of the foil and shaft surfaces leading

to high friction, wear, thermal distortion and possibly seizure or friction welding. However, experimental

NASA/TM 2002-211482 3
researchinwhichelectrical continuityacrosstheairfilmofabearingwasmeasuredhasshownthat

sporadic, localizedrubbingcontactcananddoesoccurinrunningfoilbearingsevenunderlowloadswell

belowpredictedloadcapacity(ref.16).Thiscontactmaybetheresultofunexpectedfoilmotions,

flutteringforexample, misalignment inducededgeloadingorentrappeddebris.Whateverthereason(s),

rubbingdoesoccurandif asatisfactorysolidlubricantisnotpresent, gallingandseizurecanoccur.

Toachievethemaximumpossiblefoilbearingloadcapacityasmoothsurfacefinish,properbearing

designandadequatesolidlubricationmustbeavailableatalltimesunderalloperatingconditions.For

bearingsusingPS304shaftcoatings, theseconditionsareachievedbybreakinginthebearingsurfaces

overrepeatedstart/stopoperationathightemperatures. Duringthisbreak-inperiod,thefoil surface

polishesthecoatingsurfaceandasolidlubricantrichtransferfilmdevelopsonboththefoilandshaft

resultinginabearingwithhigherloadcapacity(D- 1.0).

Unfortunately, inmanyrealapplicationstheopportunityforaprolonged, lightlyloaded,high

temperaturebreak-inperioddoesnotexist.In applicationslikegasturbineengines, maximumbearing

performanceisrequireduponfirststartingtheengine.PreviouseffortsbyBlanchetandhiscolleagues

topreconditionbearingsurfacesusingbenchlevelprocesseswereshowntobeineffective(ref.18).

Apracticalmethodisneededtoassureadequatefoilbearingperformanceinanapplicationfrominitial

installationthroughfinaluse.

Thefollowingresearcheffortconcentrates onidentifyinganddevelopingasuitablesolidlubrication

methodorsystem.Theapproachusedincludes,temporary, sacrificialcoatingsoverlaidonthePS304

shaftcoating;theuseofthin,non-galling, wearresistant ceramicandmetalfilmsappliedto foils;and

variouscombinationsofthesetribological technologies. Totesttheseconcepts, GenerationIII foil

bearingsareoperatedattemperatures from25to650°C,at14000rpmunderloadsof 10to200N as

describedinthefollowingsections.

Test Procedures and Materials

Baseline Bearing

The foil air bearings used in this study are classified as Generation III, advanced technology bearings

originally developed for an oil-free turbocharger. Their performance coefficient, D, after break-in using

PS304 shaft coatings has been found to be around 1.0 from 25 to 650 °C (ref. 19). Their geometry is

described in the patent literature (ref. 20). The extensive database available for this bearing design and

size (26 by 35 mm) provides a convenient baseline against which to judge newly developed solid

lubrication methods and materials. The bearing foils are made from Inconel X-750 sheet metal with a

typical surface finish of 0.1 microns Ra (4 micro in.). Prior to use, the bearings are cleaned in an

ultrasonic bath of ethyl alcohol and dried using clean compressed air.

PS304 Baseline Shaft Coating

The shafts are 35 mm in diameter and are plasma spray coated with a 0.35 mm thick layer of PS304

which is then heat treated in air at 650 °C for 24 hr. This heat treatment strengthens the coating and

assures dimensional stability (ref. 21). Following the heat treatment, the shafts are mounted on the test rig

spindle, shown in figure 2 and fully described in reference 8. After mounting, an in-place grinding system

is used to achieve a coating thickness of about 0.25 mm and a surface finish of about 0.4 microns Ra. To

improve this finish, the shaft is polished using silicon carbide abrasive paper with water as the polishing

coolant. PS304 coated shafts prepared in this manner have surface roughness (Ra) of about 0.2 microns.

Further polishing does not result in a smoother surface because of the porous nature of the plasma sprayed

coating.

NASA/TM 2002-211482 4
Foil Coatings

Heat cured polyimide, cathodic arc deposited aluminum bronze (A1-Cu) and sputtered alumina

(A1203) were applied to foil surfaces to improve friction and wear properties. The polyimide, described

fully in reference 22, is applied using an air spray gun, air dried at room temperature and then oven cured

at 200 °C. Several coating applications are used with burnishing (light sanding) between layers to build up

a final coating of about 0.1 mm thickness. The polyimide is commercially available and provides low

wear and friction in sliding and is capable of operating in air to about 300 °C. At higher temperatures, the

polyimide oxidizes readily and burns away.

The A1-Cu coating was applied in a vacuum chamber using a physical vapor deposition process,

which results in a hard rough coating deposit about 0.2 mm thick. This coating is polished smooth using

silicon carbide sandpaper (600 grit) with water resulting in a surface finish of about 0.1 microns Ra. The

A1-Cu coating contains about 4 percent aluminum is sometimes referred to as aluminum bronze and is

used as a common high temperature anti-fretting coating for gas turbines. It oxidizes readily in air above

400 °C forming a soft, lubricious copper oxide patina.

The Alumina, A1203, is also deposited in a vacuum chamber using a sputtering process described in

detail in reference 23. The resulting surface finish matches that of the underlying foil and requires no post

deposition polishing. Alumina is not considered a solid lubricant. Rather, it is a non-galling, wear

resistant coating which has been shown to work well in contact with PS304 in earlier tests (ref. 24).

Solid Lubricant Overlay Coatings

Fully prepared (sprayed, heat treated, ground, and polished) PS304 shaft coatings were overcoated

with either the polyimide, described in the previous section or Molybdenum Disulphide (MoS2) for

additional solid lubrication. Neither the polyimide nor MoS2 is a high temperature lubricant. Rather, they

are selected as sacrificial solid lubricants, intended to provide low friction during initial low temperature

operation. At high temperatures it is expected that these will oxidize and then volitalize (burn away). It is

hoped that by the time these overlay solid lubricants burn away, the break-in process of the underlying

PS304 coating will be complete and provide acceptable bearing load capacity performance.

The polyimide is deposited onto the PS304 surface in three layers as is done on the foils and

burnished to a thickness of about 0.05 mm. Several specimens were also prepared with thinner coatings

by increasing the amount of reducer used in the mixing of the polyimide prior to spray application. These

films were burnished to a thickness of only 0.02 mm. One benefit of using overlay coatings on the PS304

is that the overlay coatings fill in the surface pits in the PS304 resulting in much smoother shaft surfaces,

typically 0.1 micron Ra.

The MoS2 is also sprayed onto the prepared PS304 surface using an aerosol propellant in an isopropyl

alcohol vehicle. This colloidal form of MoS2 is readily available in aerosol cans from number of

manufacturers (ref. 25). Following deposition, the coating rapidly air dries at room temperature and is

polished using a soft cloth to a mirror like finish with a roughness of about 0.05 micron Ra. The polished

coating is about 0.05 micron thick.

Testing

A foil bearing load capacity test designed to simulate the conditions that may be encountered in an

oil-free aircraft engine bearing was developed to evaluate the solid lubrication materials and methods

selected in this work. Namely, upon installation in the test rig the bearings were tested for load capacity at

room temperature to simulate a cold engine start. This is followed by testing at 14 000 rpm constant speed

under about a 75 N load and increasing the temperature in approximately 100 °C increments to 650 °C.

The bearings were held at each test temperature for at least ten minutes to assure that thermal stability had

NASA/TM 2002-211482 5
beenachieved. Lift offwasconfirmedbyalowandsteadytorquetracemeasuredusingaloadcell

mountedoutsidethefurnacechamber. Loadcapacityateachtesttemperaturewasverifiedbyincreasing

theloadin10Nincrementsuntilarapidriseinfriction(torque)signaledthatthebearingloadcapacity

hadbeenreached. Additionally,followingthehightemperaturetests,theloadcapacitywasthen

measuredatroomtemperaturetodetermineif thebearingperformancewaschangedbythehigh

temperatureoperation. Thissimulates, forexample, thesecondandsubsequent flightsofanaircraft

engine.Additionally,selectedbearingsweretestedinastart/stopcyclingmodeduringheatingfrom

25to650°Ctodetermineif solidlubricantsprovidedsatisfactoryperformanceduringpureslidingover

theentireoperatingtemperaturerange.

Results and Discussion

The test results are summarized in Table II. This table incorporates the data collected in the present

study as well as comparable data published previously. The data source is clearly marked in the table.

The results corroborate earlier work in that the best bearing performance (load capacity coefficient) is

achieved when the foil and the shaft surface have good solid film lubrication characteristics, either by

transfer film formation or intentionally applied solid lubricant coatings and when both surfaces are

smooth and conform to one another (i.e., are broken in together). To better elucidate this finding, the data

is grouped by level of tribological complexity starting with the simplest case of P S304 running against an

uncoated foil, the baseline case and leading to results from bearings provided with additional lubrication

by shaft overlay and foil coatings.

PS304 Coated Shaft Running Against Uncoated Foil

This baseline case combination yields a load capacity coefficient at room temperature of only 0.3

in the as installed condition. Following sustained operation with a constant load at high temperature does

not result in significant improvements in the coefficient measured after cooling to 25 °C. During high

temperature operation the load capacity coefficient was slightly higher, about 0.5, as the result of the

formation of surface oxides on the foil and the PS304 but nowhere near the performance coefficients of

1.0 achieved following start/stop break-in. It is this loss of half of the bearing load capacity that the

current research is trying to regain through the use of additional coatings and treatments.

PS304 Coated Shaft Running Against A1203 or AI-Cu Coated Foil

The use of both of these wear resistant, non-galling foil coatings improved the load capacity

performance of the foil bearings. The load capacity coefficient for the A1-Cu was 0.5 and for the A1203

was 0.47 running against PS304 in the as-installed condition. When tested against a previously broken-in

PS304 coated shaft the coefficient for the A1-Cu rose to 0.7 and the coefficient for A1203 rose only to

0.56 about the same as for a newly installed bare foil bearing running against previously broken-in

PS304. These results are shown in figure 4. While both coatings offered improvements in the as installed

condition, the A1-Cu gave better performance.

Polyimide and MoS2 Overlay Shaft Coatings and Polyimide Foil Coatings

The results clearly show the beneficial effects of the use of traditional solid lubricants on foil bearing

load capacity. Polyimide applied to either the foil or the PS304 coated shaft and MoS2 deposited over the

PS304 dramatically improved the room temperature, as installed load capacities. Polyimide coated foils

NASA/TM 2002-211482 6
hadacoefficientof0.73whentestedagainstthepreparedPS304surface.Thisislowerthanforafully

brokeninPS304/uncoated foil case(D- 1.0)butovertwicethecoefficientforthesametestwithouthe

polyimide(D- 0.3).WhenthepolyimidewasappliedtothePS304surfaceasanoverlaythecoefficient

roseto1.25whentestedagainstanewuncoatedfoilbearing.Thisresultconfirmstheearlierassertionthat

providinglubricationtothebearingviaashaftcoatingisbetterthanrelyingonfoil coatings.Forthe

MoS2overcoatedPS304runningagainstanuncoatedfoilthecoefficientwas0.89atroomtemperature.

Thisvalueisaboutequaltoafullybrokeninbaselinebearing.

Thetruevalueoftheseoverlaycoatingscannotbeknownuntiltheircharacteristics areobserved

duringandfollowinguseathightemperatures. MoS2andthepolyimidearelowtemperaturelubricants

onlyintendedfortemporaryuse.Athightemperatures theywill oxidizeandlosetheirlubricatingability.

Whenthisoccursit isexpectedthatthelubricatingpropertiesoftheothercoatings, namelythePS304,

will developandprovideadequatesolidlubricationintheformofsurfacelayersandtransferfilms.

Tomakethistransitionpossible, theoverlaycoatingsmustdecomposeinabenignmanner.Totestthis

concept, buxn-offtestswereconductedinwhicheitherthebearingwasrunataconstant speedundera

steadyload(corresponding to aD _ 0.5) or run in a cyclic or start/stop mode under a light static load

(D _ 0.1) while the test temperature was increased from 25 to 650 °C. Both MoS2 and the polyimide lose

their effectiveness above 300 °C, oxidize and then vaporize above 500 °C. Bearing torque is monitored

during these buxn-off tests as a general indicator of performance.

The polyimide coated bearings repeatedly failed during both the continuous, moderate load bum-off

tests and during the cyclic, light load buxn-offtests. At about 350 °C, the bearing torque began to increase.

At temperatures between 400 and 537 °C, all of the test bearings seized. Upon disassembly and visual

inspection, it was observed that the bearings were jammed with large (_ 100 micron), hard particles

formed during the thermal decomposition of the polyimide. SEM analyses failed to determine the exact

chemical nature of this debris, however, the presence of silicon was noted suggesting, perhaps, the

formation of a glass-like particle. The polyimide is a commercial product and it has been known that

glass particle additives are sometimes used to control coating texture and hardness (ref. 22). Only one

polyimide coated bearing did not fail catastrophically. This bearing had most of the polyimide film

removed by polishing prior to the bum-offtest and was run continuously under a very light (D _ 0.2)

load. Though torque rose during the bum-off seizure did not occur. Clearly, in foil bearings the polyimide

does not degrade in a benign fashion.

The results with MoS2 were much more encouraging. Applying an MoS2 overlay to the prepared

PS304 surface run against an uncoated Inconel foil yielded a performance coefficient of 0.89. This value

is approximately the same as a fully broken in PS304/uncoated foil combination. During a continuous

speed bum-offtest under moderate load, no bearing torque changes or failure (seizure) occurred.

Following this successful bum-offtest, the bearing was cooled to room temperature and the load capacity

coefficient was found to be 0.54. This is lower than the previous value of 0.89 but higher than the value

for the baseline as-installed PS304/uncoated foil without start/stop break-in of 0.3. When this same

combination, PS304 + MoSz/uncoated foil, was subjected to a bum-offtest under start/stop cycles no

failure occurred and the measured coefficient after cooling to 25 °C was 0.89. Clearly, the MoS2 overlay

improves initial performance but does not adversely affect long term bearing operation.

Combined Wear Resistant Foil Coating and Sacrificial Overlay Coatings

The overall load capacity performance is significantly improved under all conditions by the use of

sacrificial overlay solid lubricants coupled with PS304 shaft coatings and non-galling, wear resistant foil

coatings like A1-Cu. For instance, when MoS2 was applied on prepared PS304 and run against an unworn

A1-Cu coated foil bearing at 25 °C the load capacity coefficient was 1.0. This is 10 percent higher than the

same shaft against a bare foil and 30 percent higher than an A1-Cu coated foil against a broken-in PS304

shaft and twice as high as the same A1-Cu coated foil running against an as prepared PS304 shaft without

an MoS2 overlay coating. Further, this apparent synergistic solid lubricant effect continues even after the

NASA/TM 2002-211482 7
burn-offtests.ThedatainTableII clearlyshowsthatthebearingcoefficientsusingboththeMoS2

overlay, PS304 shaft coatings and A1-Cu foil coatings were between 0.9 and 1.0 under all conditions

tested. This performance is better than any other solid lubricant method (coating) by itself. These results

are graphically portrayed in figure 4.

The purpose of this research effort is to overcome the diminished load capacity observed for unworn,

newly installed foil bearings which use PS304 shaft coatings. Nearly all performance data published to

date, with the exception of reference 16, gives foil bearing load capacity for bearings fully broken in

during elevated temperature start/stop cycles. Under these conditions, the PS304 and the counterface foil

surfaces self polish and develop smooth solid lubricant rich layers, which dramatically improve the load

carrying capability of the bearings. Blanchet and his co-workers have attempted to develop techniques to

achieve smoother surface finishes of P S304 and pre-develop lubricious surface layers but have been

largely unsuccessful. Their work concentrated on surface polishing and pre-wearing the PS304 shaft

coating against Inconel surfaces prior to testing. The approach taken here is different and upon fuxther

reflection is very similar to that taken in ordinary oil-lubricated sleeve bearings used, for instance, in

crankshaft bearings in automobile engines.

The hydrodynamic, oil-lubricated journal bearings found in internal combustion piston engines utilize

a complex tribological system for successful operation. This system incorporates careful, detailed

geometrical design to optimize the formation of the lubricating fluid film. The geometry of the bearing is

coupled with materials design embodied by a series of layers on the bearing surfaces to provide enhanced

performance when the oil film is insufficient to adequately separate the two surfaces. For instance, the

bearing shells are made from steel, which is then copper coated to improve heat transfer and fatigue

resistance. This copper layer is then coated with a soft metal (babbit) overlay to minimize scratching of

the shaft during starts and stops and to allow imbedding of hard debris particles that might otherwise scuff

the shaft and bearings. The shafts too are often treated to increase their surface hardness, either by chrome

plating, hard carburizing or other heat treatments. This complex lubrication system has been developed

over many decades and allows brand new bearings to operate at acceptable performance levels even

before their break-in procedure has been completed. Further, the system of lubricants and coatings

provides additional protection to the bearing to compensate for environmental conditions which exceed

nominal values such as momentary overloads, thermally induced misalignments, debris ingestion and

geometrical changes due to wear.

By treating the foil air bearing in the same fashion, a tribological system can be developed to yield

acceptable performance characteristics over the entire bearing life encompassing new installations and

fully broken in bearings near the end of their useful life. Since most engineering applications of foil air

bearings, gas turbines for example, require maximum load capacity from initial start-up it is crucial that a

systems approach to bearing lubrication be employed. For instance, it would be impractical to operate an

aircraft gas turbine at low power levels, over repeated start/stop cycles to allow gentle, gradual break-in of

the bearings. Without a satisfactory tribological system such as that demonstrated in this paper, practical

foil bearing use would be limited.

The data suggest that good performance is exhibited when wear resistant bearing surfaces are covered

with thin, smooth solid lubricants. This condition exists for well broken-in PS304/uncoated Inconel foil

bearings as well as newly installed PS304 overlaid with MoS2/A1-Cu coated foil bearings. Further, since

the use of alumina also improved the foil bearing load capacity it is likely that other materials, which

provide similar tribological functionality, will work as well. Indeed, the polyimide worked well as a room

temperature overlay coating and was only found unsuitable because it created large, hard debris during

its thermal decomposition (burn-off). Figure 5 shows a schematic representation of the tribological

"systems" used in both oil lubricated and air lubricated bearings. The use of a hydrodynamic fluid, wear

resistant materials and solid lubricants are striking similarities.

Interestingly, early pioneering efforts in rigid gas bearings by S.F. Murray postulated a similar

approach to solid lubrication (ref. 26). In his report, he pointed out that no matter how lightly loaded and

well designed the air bearing was solid contact will occur due to misalignment, thermal and centrifugal

effects, wear debris, overloads and other problems (bearing instabilities). He found that adequate solid

NASA/TM 2002-211482 8
lubricationmustbeemployedtopreventdamage. Thesameappearstobetrueforfoilbeatings.During

start/stoporunderheavysustainedloadsoccasional, localizedslidingdoesoccurandthetribological

propertiesofthecontactingsurfaceshaveadramaticeffectonbearingloadcapacityperformance.

Concluding Remarks

The results of this research clearly indicate that at least three conditions must be met to achieve good

load capacity of foil air bearings; proper geometrical design, smooth surface finish and adequate solid

lubrication and wear resistance. If any of these are not met bearing performance or life suffers. Early

work utilizing PS304 as the sole source of solid lubrication yielded good results only after repeated high

temperature start/stop operation encouraged the formation of smooth surfaces and lubricious transfer

films on both the foils and the shaft. In order to achieve good performance upon initial installation it

was shown that an effective solid lubricant film, such as MoS2, must be present on the shaft or foil.

Non-galling wear resistant coatings such as PS304 on the shaft and A1-Cu on the foil enhances

performance even more. To prevent bearing failure, the materials selected must not produce large hard

debris particles as was the case for the polyimide, especially upon thermal decomposition (buxn-off).

It appears that the combination ofPS304 shaft coatings with a sacrificial layer of MoS2 running

against A1-Cu coated foils works synergistically to give maximum load capacity from first installation to

fully run-in foil bearing operation. These results are expected to be relevant to other highly loaded foil air

bearing designs and geometries and solid lubricants. Based upon the findings presented, oil-free

turbomachinery using foil bearings have promise as practical systems.

References

1. Blok, H.; and vanRossum, J.J.: The Foil Bearing-A New Departuxe in Hydrodynamic Lubrication.

ASLE, Lubrication Engineering, pp. 316 330, December 1953.

2. Ma, J.T.S.: An Investigation of Self-Acting Foil Bearings. ASME, Journal of Basic Engineering,

pp. 837 846, December 1965.

3. Barnett, M.A.; and Silver, A.: Application of Air Bearings to High Speed Turbomachinery. SAE

Paper 700720, September 1970.

4. Gross, W.A.: Gas Film Lubrication, John Wiley and Sons, Inc., 1962.

5. Heshmat, H.; Shapiro, W.; and Gray, S.: Development of Foil Journal Bearings for High Load

Capacity and High Speed Whirl Stability. ASME Journal of Lubrication Technology, vol. 104, no. 2,

pp. 149 156, 1982.

6. DellaCorte, C.; and Wood, J.C.: High Temperature Solid Lubricant Materials for Heavy Duty and

Advanced Heat Engines. NASA TM 106570, September 1994.

7. Wagner, R.C.; and Sliney, H.E.: Effects of Silver and Group II Fluorides Addition to Plasma Sprayed

Chromium Carbide High Temperature Solid Lubricant for Foil Gas Bearings to 650 °C. NASA

TM 86895, 1985, ASLE, Lubrication Engineering (42), vol. 10, pp. 594 600, 1986.

8. DellaCorte, C.: Composition Optimization of Cr3C2 Based Solid Lubricant Coatings for Foil Gas

Bearings at Temperatures to 650 °C. NASA CR179649, July 1987.

9. DellaCorte, C.; and Edmonds, B.J.: Preliminary Evaluation ofPS300: A Self-Lubricating High

Temperature Composite Coating for Use to 800 °C. NASA TM 107056 (199).

10. DellaCorte, C.; and Fellenstein, J.A.: The Effects of Compositional Tailoring on the Thermal

Expansion and Tribological Properties of PS300: A Solid Lubricant Composite Coating. NASA

TM 107332, 1996.

11. DellaCorte, C.: The Evaluation of a Modified Chrome Oxide Based High Temperature Solid

Lubricant Coating for Foil Gas Bearings. NASA/TM 1998-208660.

NASA/TM 2002-211482 9
12.DellaCorte, C.;Valco,M.J.;Radii,K.C.;andHeshmat, H.:PerformanceandDurabilityofHigh

TemperatureFoilAirBearingsforOil-FreeTurbomachinery. NASA/TM_000-209187.

13.Emerson, T.P.:TheApplicationofFoilAir BearingTurbomachinery inAircraftEnvironmental

ControlSystems. ASMEProceedingsoftheIntersocietyConferenceonEnvironmental Systems, San

Diego,CA,July10 13,1978,Papernumber780ENAS18.

14.CapstoneMicroturbineCorporation, Chatsfield, CA,pressrelease, 2000.

15.Heshmat, H.;WaltonIII, J.F.;DellaCorte, C.;andValco,M.J.:Oil-FreeTuxbocharger PavestheWay

toGasTurbineEngineApplications. ASME2000GT 620,2000.

16.Radii,K.C.;andDellaCorte, C.:TheEffectofJournalRoughnessandFoilCoatingsonthe

PerformanceofHeavilyLoadedFoilAirBearings. NASA/TM_000-210941.

17.DellaCorte, C.;andValco,M.J.:LoadCapacityEstimationofFoilAir JournalBearingsforOil-Free

Tuxbomachinery Applications. NASA/TM 2000-209782, andSTLETribologyTransactions, 43(1),

October2000.

18.Kim,Jong-Hwan: SurfaceRoughness, Tribology,StressAnalysisof Thermal-Sprayed Composite

CoatingsforFoilGasBearings. Ph.D.Dissertation, Rensselaer PolytechnicInstitute,Troy,NY,

October2001.

19.DellaCorte, C.:A NewFoilAir BearingTestRigforUseto700°Cand70,000rpm.STLETribology

Transactions, vol.41,no.3,1998,pp.335340.

20.Heshmat, H.:HighLoadCapacityJournalFoilBearing.U.S.Patent#5,988,885, November1999.

21.DellaCorte, C.;Edmonds, B.J.;andBenoy,P.A.:ThermalProcessingEffectsontheAdhesive

StrengthofPS304HighTemperatureSolidLubricantCoatings. NASA/TM_001-210944.

22.Wagner,R.C.:Experimental TestProgramforEvaluationofSolidLubricantCoatingsasAppliedto

CompliantFoilGasBearingsto 315°C.NASACR 17483,1985.

23.Lei,J.F.:AdvancesinThinFilmSensorTechnologiesforEngineApplications. ASME97 GT 458,

ASME,NY(1997).

24.DellaCorte, C.;Fellenstein, J.A.;andBenoy,P.A.:EvaluationofAdvancedSolidLubricantCoatings

forFoilAir BearingsOperatingat25and500°C.STLETribologyTransactions, vol.42,no.2,

pp.338342,April1999.

25.AchesonColloidsCompany, PortHuron,MI.

26.Murray,S.F.:ResearchandDevelopment ofHighTemperatureGasBearings. VolumeII,NASA

CR MTI TR22,March1969.

Table I. Materials

Component/Coating Composition/Characteristic Function/Attributes

Foil material lnconel X-750 High temperature spring properties

Shaft material lnconel 718 High temperature strength

PS304 coating 60 NiCr, 20 Cr203, 10 Ag, 10 BaF2/CaF2 Wear resistant solid lubricant

Alumina coating A1203 Wear resistant sputtered film

Aluminum bronze coating A1-Cu (4 to 96 percent) Wear resistant cathodic arc deposited

Polyimide coating Organic polymer Sacrificial solid lubricant air spray heat-cured

polymer coating

MoS2 MoS2 in an alcohol carrier Sacrificial solid lubricant coating

NASA/TM 2002-211482 10
Table ll._ata

Shaft surface Foil surface Type of test

PS304 as-prepared Uncoated-unworn Load capacity,

30 Krpm

PS304 broken-in Uncoated-unworn Load capacity,

30 Krpm

PS304 broken-in Uncoated-broken-in Load capacity,

30 Krpm

PS304 as-prepared A1203 unworn Load capacity,

30 Krpm

PS304 broken-in A1203 unworn Load capacity,

30 Krpm

PS304 as-prepared A1Cu unworn Load capacity,

30 Krpm

PS304 broken-in A1Cu unworn Load capacity,

30 Krpm

PS304 as-prepared Polyimide coated foil Load capacity,

30 Krpm

PS304 broken-in Polyimide coated foil Load capacity,

30 Krpm

PS304 polyimide coated Uncoated-unworn Load capacity,

14 Krpm

PS304 MoS2 coated Uncoated-unworn Load capacity,

14 Krpm

PS304 polyimide coated Uncoated-unworn Burn-off/

Constant speed

PS304 thin polyimide coated Uncoated-unworn Burn-off/

Constant speed

PS304 MoS2 coated Uncoated-unworn Burn-off/

Constant speed

PS304 MoS2 coated Uncoated-unworn Burn-off/

Start-stop

PS304 MoS2 coated A1Cu unworn Load capacity,

14 Krpm

PS304 MoS2 coated A1Cu unworn Burn-off/

Constant speed

PS304 MoS2 coated A1Cu worn Burn-off/

Start-stop

PS304 MoS2 coated/worn A1Cu worn Extended burnoff/Staxt- stop

PS304 MoS2 coated A1Cu worn Load capacity,

14 Krpm

D Data source/Comments

0.33 25 °C, as-installed load capacity (ref. 16)

0.56 25 °C, new bearing, worn shaft (ref. 16)

-1.0 25 to 650 °C, fully broken in bearing/

shaft (ref. 19)

0.47 25 °C, as-installed load capacity (ref. 16)

0.56 25 °C, new bearing, worn shaft (ref. 16)

0.50 25 °C, as-installed load capacity (ref. 16)

0.70 25 °C, new bearing, worn shaft (ref. 16)

0.73 25 °C, as-installed load capacity (ref. 16)

1.05 25 °C, new bearing, worn shaft, (ref. 16)

1.25 25 °C, as-installed load capacity

0.89 25 °C, as-installed load capacity

-- Seizure above 400 °C

-- Light load, torque rose but no seizure

0.54 D measured at 25 °C following burn-off

0.89 D measured at 25 °C following burn-off

1.0 25 °C, as-installed load capacity

0.9 D measured at 25 °C following burn-off

1.0 D measured at 25 °C following burn-off

1.36 D measured at 25 °C following extended

start/stop

1.05 25 °C, prewom bearing, new shaft load

capacity

NASA/TM 2002-211482 11
Leaf foil

I

/_ Bearing sleeve Bump foil_ /_ Bearing sleeve

/_ Journal /_ Journal

(b)

Figure 1 .--Schematic example of typical first generation foil bearings with axially and circumferentially uniform

elastic support elements. (a) Leaf-type foil bearing. (b) Bump-type foil bearing.

(a)

Bearing

sleeve _ /

Figure 2.--Schematic of Generation III foil air bearing

tested.

NASA/TM 2002-211482 12
>

I

oo

c.

O

Force

©

Counterweight

Front

view

i

©' i _>

I_//-- Torque rod

Bearing II

sleeve --, I I

.... Jl

\

\

Force

Torque transducer

LVDT load cell

/--Bearing

/ journal

/-- Quartz

,/ tubes

_ Bearing

housing

deadweight

\\

"-- Furnace (left

half removed

for clarity)

Stand

Torque

transducer

\\

Test rig side view

Removable

heat box

Quartz lamp heaters

Removable

test

journal

Test

bearing and

floating /

housing J

Radial load

\

_- Spindle

bearing

housing

_ Deadweight

loading cable

Figure 3.--Foil bearing test rig and bearing set-up.

Fiber

optic

rpm

probe

/

Z-Belt to

electric

motor
1,0 m

E

v

ttO.m

0

0

Q.

0

_J

0.5

0.0

Foil --_- Uncoated IX 750 Uncoated IX 750

Shaft --,_ PS304 and PS304

MoS2

Solid lubrication system

AI-Cu

PS304

AI-Cu

PS304 and

MoS2

Figure 4.--Load capacity coefficient of as installed (not broken-in) foil bearing at

25 °C, showing the synergistic effects of wear resistant (AI-Cu) foil coatings, self

lubricating shaft coatings (PS304) and sacrificial overlay solid lubricants (MoS2).

NASA/TM 2002-211482 14
Rigid

housing

\

\

Steel sleeve \\

bearing

r_ Copper layer

i (heat transfer)

I

I

i r-- Babbit

i _ (soft solid tube)

/

Rigid

housing

\

\

Bump foil \

(elasticity)

Air

r_ Top foil

i (traps air)

I

/

/ _ Oil (hydrodynamic

/ fluid)

L_ Chrome plate

or carbonized layer

\

/

/ _AI - Cu

L__ PS304 (wear

coating resistance)

_ Hard layer Shaft

Copper _ "'- Oil fluid _ _/._ Soft babbit

(a)

PS304

Superalloy

top foil _.

Superalloy

elastic

foundation

Shaft /_ M°S2

\ / overlay

Air fluid - Cu

(b)

Figure &--Schematic representation of systems approach to bearing lubrication (a) convential oil lubricated

bearing, (b) multilevel solid/gas lubricated foil air bearing.

NASA/TM 2002-211482 15
Form Approved REPORT DOCUMENTATION PAGE

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

October 2002 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

A Systems Approach to the Solid Lubrication of Foil Air Bearings for

Oil-Free Turbomachinery

6. AUTHOR(S)

Christopher DellaCorte, Antonio R. Zaldana, and Kevin C. Radil

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546 0001

and

U.S. Army Research Laboratory

Adelphi, Maryland 20783 1145

WU-708-18-13-00

1L161102AH45

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-13250

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2002-211482

ARL-TR-2867

11. SUPPLEMENTARY NOTES

Christopher DellaCorte and Antonio R. Zaldana, NASA Glenn Research Center; Kevin C. Radil, U.S. Army Research

Laboratory, NASA Glenn Research Center. Responsible person, Christopher DellaCorte, organization code 5960,

216-433-6056.

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Unclassified - Unlimited

Subject Category: 23 Distribution: Nonstandard

Available electronically at http://gltrs.arcmasa.gov

This publication is available from the NASA Center for AeroSpace Information, 301-621-0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

Foil air bearings are self-acting hydrodynamic bearings which rely upon solid lubricants to reduce friction and minimize

wear during sliding which occurs at start-up and shut-down when surface speeds are too low to allow the formation of a

hydrodynamic air film. This solid lubrication is typically accomplished by coating the non-moving foil surface with a

thin, soft polymeric film. The following paper introduces a systems approach in which the solid lubrication is provided by

a combination of self lubricating shaft coatings coupled with various wear resistant and lubricating foil coatings. The use

of multiple materials, each providing different functions is modeled after oil-lubricated hydrodynamic sleeve bearing

technology which utilizes various coatings and surface treatments in conjunction with oil lubricants to achieve optimum

performance. In this study, room temperature load capacity tests are performed on journal foil air bearings operating at

14 000 rpm. Different shaft and foil coating technologies such as plasma sprayed composites, ceramic, polymer and

inorganic lubricant coatings are evaluated as foil bearing lubricants. The results indicate that bearing performance is

improved through the individual use of the lubricants and treatments tested. Further, combining several solid lubricants

together yielded synergistically better results than any material alone.

14. SUBJECT TERMS

Lubrication; High temperature; Coatings

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Unclassified

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21

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